The present invention relates generally, to an improved method of and apparatus for pumping a laser and, more specifically, to the novel use of a Nernst lamp to pump a solid state laser.
At the present time tungsten-iodine incandescent lamps are the most popular choice for pumping medium power solid state cw lasers. The spectral distribution of radiant energy of a tungsten-iodine lamp is essentially a scaled down black body distribution. Consequently, only a small fraction of the input power appears as emitted energy in the region of the absorption spectrum of a laser material, most of the energy being emitted in the infrared. Another problem associated with tungsten lamps, which lower the overall efficiency of a laser system is an unstatisfactory optical coupling between the lamp and the laser due to a poor geometrical configuration formed by the lamp with the laser rod in an optical cavity.
In general, in order to improve the luminous efficiency, or fraction of energy emitted in the visible by an incandescent source, one can: (1) operate the source at a higher temperature, or (2) make the light emitting elements from a material with characteristics which deviate from those of a black body and which emit radiation selectively in the pump region of the spectrum. The first approach is unacceptable since it leads to a drastically shortened lamp life. The second approach, which until now has been overlooked in the laser pump application, is the subject matter of the present invention and involves the use of selective thermal radiators, commonly called Nernst lamps.
Selective thermal radiators are ceramics which are made of mixtures of metallic oxides with low and high thermal emissivities in the visible region. By nature they are conductors whose electrical conductivity is a negative function of temperature. When heated to incandescence, selective radiators reach a higher temperature than a black body heated with the same input power because they emit radiation discretely in the selected region of the spectrum. By a proper choice of mixture composition it is possible to concentrate emission in the region of primary absorption of a laser material. Selective thermal radiators outperform a black body radiator since they reach a higher temperature than the black body for a given amount of power supplied, and will radiate a larger fraction of energy in the visible. The mixture is desirable in order to obtain higher temperature. The low emissivity material will reach a high temperature when power is supplied to it, but because of its low emissivity throughout the visible spectrum little visible radiation results. A material having a high radiant emittance in the visible and a low emittance elsewhere will radiate more in the visible but will not reach as high as temperature for a given amount of power supplied. By mixing a small amount of the two materials, the amount of visible power radiated can be maximized; the material will reach a high temperature and will also radiate in the visible.